1. FIELD OF THE INVENTION
The present invention relates to a nickel/hydrogen storage battery in which is used a hydrogen absorbing alloy capable of carrying out the electrochemical hydrogen absorbing and desorbing reaction, as a negative electrode, and more particularly to improvement of its battery characteristics.
2. DESCRIPTION OF RELATED ART
A lead storage battery and a nickel/cadmium storage battery (hereinafter referred to as a "Ni-Cd battery") have been put to practical use at present and widely used in portable apparatuses. The lead storage battery is inexpensive but generally low in energy density per unit weight (Wh/kg) and has problems in cycle life and is not suitable as an electric source for portable apparatuses of a small size and light weight.
On the other hand, the Ni-Cd battery is higher than the lead storage battery in energy density per unit weight and superior in reliability of cycle life and hence it is widely used as an electric source for various portable apparatuses.
However, novel secondary batteries having the reliability similar to that of the Ni-Cd battery and high in energy density have been desired as an electric source for portable apparatuses. Recently, a nickel/hydrogen storage battery in which is used as a negative electrode a hydrogen absorbing alloy capable of carrying out the electrochemical absorbing and desorbing reaction (charging and discharging reaction) of hydrogen which is an active material of a negative electrode (hereinafter referred to as a "hydrogen absorbing alloy negative electrode") in place of the cadmium negative electrode of the Ni-Cd battery has been noticed as a novel secondary battery of high capacity.
Since the hydrogen absorbing alloy negative electrode is higher than the cadmium negative electrode in energy density per unit volume, when a battery having a constant internal volume and controlled in positive electrode capacity is constructed using the hydrogen absorbing alloy negative electrode, this battery has a higher battery capacity than that of the Ni-Cd battery due to the increase of volume of a nickel positive electrode.
As explained above, the nickel/hydrogen storage battery can be expected to be enhanced in capacity than the Ni-Cd battery but has a problem in cycle life charactreristics. This problem is caused by
(1) Since materials to constitute the battery, such as the active material of the positive electrode and etc., are filled in a battery case of a given volume, in a larger amount than in the conventional batteries, the space volume of the battery is decreased and it is impossible to add an electrolyte necessary for satisfying the above characteristics.
(2) Owing to the increase in capacity of the nickel positive electrode, the current density increases at the same charging rate as in the Ni-Cd battery. As a result, the nickel positive electrode is more ready to produce a .gamma.-type nickel oxyhydroxide having a higher expansion coefficient than that of a .beta.-type nickel oxyhydroxide at the time of charging, than the conventional Ni-Cd battery. Due to this expansion of the positive electrode, a separator is more easily compressed in the nickel/hydrogen storage battery than in the conventional Ni-Cd battery. Therefore, as compared with the conventional Ni-Cd battery, the electrolyte in the separator and negative electrode is absorbed in the positive electrode by repetition of charging and discharging cycle and the distribution of the electrolyte in the battery changes and thus the internal resistance is apt to increase.
These problems are also seen in the Ni-Cd battery which has been increased in capacity, and generally in order to reduce the expansion of the positive electrode occuring due to repetition of charging and discharging in the Ni-Cd battery, cadmium oxide or cadmium hydroxide is added to the positive electrode to inhibit the production of the .gamma.-type nickel oxyhydroxide at the time of charging. Furthermore, in order to control the expansion of the positive electrode, it has been proposed to add calcium hydroxide to the surface of the nickel positive electrode (Japanese Patent Kokai (Laid-Open) No. Sho 62-66569) and to add zinc, zinc oxide and zinc hydroxide together with the positive electrode active material (Japanese Patent Kokai (Laid-Open) No. Sho 59-83347).
When an enclosed nickel/hydrogen storage battery is constructed by mere combination of the nickel positive electrode proposed for the Ni-Cd battery as mentioned above with the hydrogen absorbing alloy negative electrode, there is a problem that most of the electrolyte migrates into the positive electrode due to expansion of the positive electrode to cause change in distribution of the electrolyte in the battery and increase of the internal resistance and, as a result, such battery is inferior in cycle life characteristics to the Ni-Cd battery which is now practically used. When a nickel positive electrode containing cadmium oxide, calcium hydroxide, zinc oxide or the like is used, the expansion of the positive electrode is inhibited also in the nickel/hydrogen storage battery, but the inhibition effect is not so high as in the Ni-Cd battery because the nickel/hydrogen storage battery is higher in capacity than the Ni-Cd battery. Besides, the cadmium oxide added to the positive electrode is partially dissolved in an alkaline electrolyte and, hence, the cadmium oxide is precipitated on the alloy surface of the hydrogen absorbing alloy negative electrode upon repetition of charging and discharging. This causes the reduction of the absorbing ability of the hydrogen absorbing alloy to result in further the reduction of cycle life.